In a manufacturing process of an organic light emitting diode (OLED) display, an evaporation process is generally used to form an organic light emitting layer. According to the evaporation method in the prior art, the organic light emitting layer has a cross-sectional shape with a relatively thick middle area and relatively thin side areas. This enables the organic light emitting layer to have a non-uniform thickness. For a single pixel formed, this may result in different light emitting brightness in different areas. While for a plurality of pixels, this may result in problems such as color mixture (or color bias).